Reduced urinary kallikrein is associated with human essential hypertension and with genetic hypertension in Dahl salt-sensitive (S) rats. We have purified a related enzyme, esterase A2 from the urine of Dahl salt-resistant (R) rats. Esterase A2 is absent from the urine of S rats, but it is replaced in S rats by yet another enzyme which we call esterase A3. The biological properties of esterase A3 are unknown. Esterase A2, like kallikrein, releases kinins which in the kidney cause vasodilation and increased sodium excretion. Reduced renal kinin generating activity in S rats may, therefore, exacerbate hypertension by potentiating sodium retention. We have also found that the urine of S rats contains two proteinases inhibitors, alpha1-proteinase inhibitor (Alpha1-PI) and kallikrein binding protein-1 (KBP-1). Partially purified KBP-1 inhibited the kinin generating activity of both kallikrein and esterase A2. Alpha1-PI inhibited only esterase A2. The origins of the urinary esterases A2 and A3 and the urinary inhibitors are not known. The aims of the proposed research are to (1) purify KBP-1 to homogeneity, 2) quantitate KBP-1 in the urine of S and R rats and determine the proportion free or bound to enzymes, 3) determine if the kidney synthesizes the Alpha1-PI and KBP-1 present in urine, 4) purify urinary esterase A3 from S rat urine, 5) determine if esterase A3 generates vasopressor or vasodilator activity, 6) compare the physiochemical properties of esterase A2 and esterase A3, 7) isolate rat-plasma low-molecular-weight kininogen and determine the kinin-generating activity of kallikrein, esterase A2 and esterase A3 from this natural substrate and 8) prepare monoclonal antibodies against rat urinary kallikrein and rat urinary esterase A2 for use in immunohistochemical localization of these enzymes in the kidney.